Fix #plugin action not working in modern versions of Sketch

[rodionovd]

Getting this action working again is key to unlock workflows like this one: https://forum.sketch.com/t/populate-color-from-json-file/2465

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P.S. The official documentation lacks practical examples so here's how the #plugin action is supposed to be used:

1. Let's say you have the following plugin installed that you want to invoke on one of your template layers based on some condition:
   

2. Let's also assume that you have the following template layer structure:
   

3. Now, to invoke the plugin on the Rectangle layer, rename the latter as follows:

   #plugin[{condition}, My Plugin > My Action]
   

   

   ℹ️ There're no quotation marks around the plugin command path.
   ℹ️ You may only invoke single-level commands, so things like My Plugin > Group > My Action won't work).

Notes

• Your {condition} might be as simple as just true (e.g. #plugin[true, My Plugin > My Action]) if you want to always invoke the plugin no matter what's in your data. It may also reference other variables in your data: see examples.

• You may pass arguments to a plugin by including them in a layer name:

  #plugin[{condition}, My Plugin > My Action, arg1, arg2 {zip_code}, arg3]
  

  ℹ️ The {zip_code} placeholder in the example above will be replaced with an actual value from your data just like any other placeholder).

  In order to access those arguments in your plugin, use the following:

  // in script.js
  
  const util = require('util')
  var onRun = function (context) {
     let arguments = util.toArray(context.selection[0].userInfo()["datapopulator"]).map(String)
     // arguments == ["arg1", "arg2 {94024}", "arg3"]
  }

  As you can see, argument are passed to the plugin as an array of strings.

  ⚠️ Don't forget to use util.toArray() and the map bit to convert it to a proper JavaScript array of Strings.

[rodionovd]

Linking #220

[rodionovd]

In theory we might use the context argument here to pass the same command params as we do via userInfo, but it will suffer from the same serialization limitations, so I wouldn't bother.